The present invention relates to a drive unit for a removable medium type disc such as a CD or a DVD.
JP-A-2003-168282 (Refer to page 2 and FIG. 3) discloses a disc drive unit as a conventional example. This disc drive unit is adapted to aim at reducing deformation of a disc in a deformation mode at its critical speed. It is noted here that the critical speed is the one at which a natural frequency of backward rotating vibration traveling in a direction reverse to a rotating direction of the disc becomes 0 Hz as viewed externally. If the rotating system is viewed externally, the deformation of the disc moderately drifts around this rotating speed. The disc drive unit disclosed in the JP-A-2003-168282 (Refer to page 2 and FIG. 3) has a disc tray formed therein with a groove for feeding an optical head, which has a groove width set to be not less than a predetermined width in order to induce an incommensurate air distribution in the drive unit. With this configuration, a force which causes excitation of deformation is cancelled out so as to avoid increasing the amplitude of vibration.
In order to enhance the performance of recording and reproduction of a disc drive unit in a CD-ROM or DVD apparatus, it is indispensable to drive the disc unit at a high speed including a critical rotating speed. At this critical speed, the rotation of a disc is unstable, and sensitively responds to affection by disturbance so as to cause deformation and vibration. It had been better to drive the disc drive unit at a speed other than the critical rotating speed lin order to avoid occurrence of this problem. However, at present, the speed-up of CD or DVD apparatus has been advanced, The rotating speed is changed in accordance with a position of an optical head over a speed control range which includes the critical speed.
Further, in a disc drive unit capable of writing data on a CD-R, a DVD-R or the like, detrimental affection such as deformation or vibration becomes especially higher during writing operation thereof than during reading operation. In particular, the inventors have found such a fact that should a node of disc deformation be present right above an optical head, the surface of the disc would be largely inclined so as to greatly lower the performance of recording and reproduction.
No sufficient consideration has been made as to lowering of the stiffness of a disc tray part in the disc drive unit disclosed in the JP-A-2003-168282 (page 2 and FIG. 3). That is, should the width of the groove formed in the disc tray part be too large, the stiffness of the disc tray part would be lowered, possibly causing such a risk that the disc tray vibrates due to an air stream induced by rotation of the disc. Further, through observation of a behavior of deformation at the critical speed at an optical head side (the absolute static system, it has been found that the deformation of the disc is steady state, or any position on the disc is always deformed in one and the same direction, like a DC component in a modulated carrier wave. Thus, it is important to consider a deformation mode above the optical head, in comparison with reduction of the amplitude of vibration of a disc. However, this consideration has not yet been made in this drive disc unit.